The invention relates to a magnet coil arrangement, in particular for operating valves.
A magnet coil arrangement of this type is disclosed by DE 43 41 087 C2. A coil former which is provided with an electric winding is arranged in a metal housing. A plug-in base provided with contact tabs is held on the metal housing by a metal connecting part. One of the contact tabs is connected to the metal housing via the connecting part as a protective conductor connection. The ends of the coil windings are connected to two further contact tabs. The free spaces between the wound coil former, the metal housing and the plug-in base are filled with plastic. Moisture can penetrate into the magnet coil arrangement through gaps at the interfaces between the metal housing and the plastic. In order to prevent the moisture entering the connecting space through the plug-in base along the contact tab serving as the protective conductor connection, a sealing ring which encloses the connecting part is provided between the contact tab serving as protective conductor connection and the metal housing. During the assembly preceding the encapsulation, the connecting part is pressed into a recess in the metal housing and into a recess in the contact tab. In the process, the sealing ring is clamped in between the metal housing and the contact tab.
Because of the curvature of the metal housing, the distance between the contact tab and the metal housing in the contact area of the sealing ring is of variable size, so that the sealing ring gives way outwardly to a greater or lesser extent. This leads to nonuniform deformation of the sealing ring over its circumference. The free circumferential surface of the sealing ring, that is to say the area of the sealing ring that is not resting on a metal part comes into contact with the plastic during encapsulation. As a result of the high temperature of the plastic during the encapsulation, the material of the sealing ring is highly thermally stressed. In addition, it is not ensured that, following the curing of the plastic, during which a certain shrinkage always has to be expected, a satisfactory sealing effect between the plastic and the surface of the sealing ring which is free before the encapsulation is ensured. There is therefore the risk that moisture which passes to the sealing ring via a gap between metal housing and the plastic, via a gap between the sealing ring and the plastic adjoining it and via the gap between the contact tab and the plug-in base or via the gap between the contact tab and the plastic, will nevertheless penetrate into the connecting space.
The invention is based on the object of providing a magnet coil arrangement of the type mentioned at the beginning which prevents liquid which has penetrated into the magnet coil arrangement via a gap between the metal housing and the plastic adjoining the latter being passed on into the connecting space.
In the magnet coil arrangement according to the invention, the sealing part touches neither the contact tab serving as a protective conductor connection nor the metal housing. Since the contact tab runs within the plug-in base in the area in which the connecting part engages in said contact tab, it is sufficient for the sealing part to rest on the connecting part on one side and on the plug-in base on the other side. Since the sealing part has a force applied to it even before the encapsulation of the magnet coil arrangement, the pressure of the plastic during encapsulation and in the following cooling phase plays no part in the sealing effect. In addition, the shrinkage of the plastic during cooling does not play any part in the sealing effect. The sealing part can be arranged between the connecting part and the plug-in base so as to seal both radially and axially.
If the sealing part is arranged to seal radially, it is advantageous to arrange the sealing part in a recess in the plug-in base which is matched to the outer contour of the sealing part, the recess is deeper than the thickness of the sealing part, the sealing part can give way in the axial direction that is to say in the installed state it assumes a substantially oval to egg-shaped cross section. A shoulder on the connecting part ensures that there is a defined distance between the metal housing and the surface of the plug-in base that faces it. If the outer diameter of the shoulder is greater than the inner diameter of the recess, no plastic reaches the sealing part during the encapsulation of the magnet coil arrangement. In addition, the part of the shoulder resting on the plug-in base prevents the connecting part being pulled out of the plug-in base under high tension. As an alternative to a radially sealing arrangement of the sealing part, the invention permits an axially sealing arrangement of the sealing part. In this case, the connecting part is provided with a collar which, when the connecting part is pressed into the contact tab held in the plug-in base, comes into contact with the plug-in base and in this way limits the force acting on the sealing part. In addition, the collar prevents the sealing part coming into contact with the hot plastic during encapsulation if the connecting part is rotationally symmetrical, at least in the sealing area, a commercially available and therefore cost-effective O ring can be used as the sealing part. If one side face of the plug-in base is provided with a projection between the outwardly pointing side and the side of the plug-in base that faces the metal housing, the result is a form-fitting connection which, even when there is high tension on the contact tabs or on the connecting part, prevents the plug-in base being pulled out of the plastic body. One or more recesses on the side of the plug-in base which faces the metal housing, said recesses surrounding the connecting part, improve the sealing effect in the boundary area between the plug-in base and the plastic used for the encapsulation, the recesses making initial melting easier during encapsulation or being effective as a labyrinth seal. If only that section of the contact tab serving as a protective conductor connection in which the connecting part engages is held in the plug-in base, the production of the plug-in base is simplified. Following the production of the plug-in base, the contact tab is pushed into a recess in the plug-in base. In this case, it is advantageous to arrange this recess at right angles to the recess provided for the connecting part. Even better sealing against penetration of moisture into the connecting space may be achieved if the contact tab serving as a protective conductor connection is held only in the plug-in base.